See-through displays comprise three fundamental parts: an image projector, a computing unit controlling the projector, and an optical combiner which is adapted to show the image from the projector to a see-through display, which allows the user of the display to see both the scenery behind the display and the projected image without requiring the user to look away from his usual viewpoint. In a diffractive grating based see-through display solutions image is coupled from an in-coupling grating into a waveguide. From the waveguide the image is coupled out to the eye of a user by an out-coupling grating. Several examples of devices operating with this general principle are introduced in WO 2006/064301, WO 99/52002, WO 2009/077802, WO 2009/077803, WO 2011/110728 and US 2009/0245730. Diffractive gratings are sometimes referred to as holographic optical elements (HOEs).
In some solutions, two or more gratings are used to diffract different wavelengths, serving as so-called multiplex diffraction gratings. For example, WO 2011/113662 discloses a diffractive combiner for a color head-up display device. The device includes a first optical diffraction grating adapted for diffracting, in a diffraction direction, light having a first wavelength and which is incident on the first grating in an incidence direction, a second optical diffraction grating adapted for diffracting, in the same direction, light having a second wavelength and which is incident on the second grating in the incidence direction. The first and second optical diffraction gratings are formed as reliefs on first and second opposite surfaces of the combiner.
In some solutions, there is a plurality of gratings with different orientations. For example U.S. Pat. No. 4,856,869 discloses a display element including a substrate and a display pattern formed on the substrate, the display pattern having a first diffraction grating structure and a second diffraction grating structure. The direction of the grating lines of the first diffraction grating structure differs from that of the grating lines of the second diffraction grating structure, thereby aiming at preventing the occurrence of a rainbow-like effect in the image.
A more efficient solution for preventing the rainbow effect is disclosed in WO 2014/044912. It is based on reducing the amount of light diffracted to non-zero transmission orders from the grating using first and second grating layers comprising periodically alternating zones having different refractive indices. To achieve this, the zones of the first grating layer having higher refractive index are at least partly aligned with the zones of the second grating layer having lower refractive index and vice versa. As a result, the diffraction of transmissive light is decreased. This solution suffers from lower out-coupling efficiency compared with solutions with a single grating per wavelength.
Besides the out-coupling efficiency, another important factor in see-through displays is the transparency of the out-coupling grating. In particular, in wearable displays, such as head-mounted displays (HMDs), the grating is close to the eye and low transparency reduces the brightness of the scenery behind the display. WO 2014/044912, discloses a solution which utilizes a thin metallic layer in the out-coupling grating structure for boosting out-coupling efficiency. This metallic layer, however, decreases the transparency of the grating. A loss of transparency highly affects the user experience and limits or prevents the use of the device in detailed work and driving of a car, for example. Also a darker area over the eye results in a less consumer attractive look of the device.
There is also a plurality of other kinds of diffractive gratings designed for various purposes. For example, WO 2006/132614 discloses a device comprising in-coupling and out-coupling diffractive gratings. There is also an intermediate diffraction grating arranged laterally with respect to the in- and out-coupling gratings in an angle so as to couple diffracted light component from in-coupling grating to the out-coupling grating.
An article by Saarikko P., Diffractive exit-pupil expander with a large field of view, Photonics in Multimedia II (2008), Volume 7001, discloses a concept of asymmetric exit-pupil expansion for head-worn virtual displays. US 2010/296163 presents a related apparatus and method for providing a wide field-of-view as well as illumination uniformity in exit pupil expanders (EPE) using stacked EPE substrates with non-symmetric exit pupil expansion that use a plurality of diffractive elements for expanding the exit pupil of a display for viewing. The solution aims at high field of view (FOV) and illumination uniformity.
In diffractive display elements based on light-guides that have a uniform out-coupling grating, the brightness of out-coupled light decreases towards the rear end of the light-guide. Attempted solutions to his problem include trying to provide an out-coupling efficiency that increases towards a rear end of a light-guide. Fabrication of grating profiles with a changing grating depth is however challenging and expensive.
To summarize, in the prior art available, there are solutions with gratings on top of each other for the purposes of improving the response of the devices to different wavelengths and preventing the occurrence of a rainbow-like effect in the image. None of the publications, however, provide as high brightness uniformity and white balance for the image to be displayed and/or as high transparency for the through-coming light and/or as small side effects, such as rainbow effect or other kind of deterioration of image quality, as would be desirable.
Thus, there is a need for improved see-though displays.